cannot write to the registry key

Try this:

RegistryKey skms = SoftwareKey.OpenSubKey(RegistryKeyName, true);

The second parameter should be set to true if you need write access to the key.

-EDIT-

On 64-bit system, you can try this (if you are using .Net 4):

private readonly RegistryKey SoftwareKey = 
    RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry64).
    OpenSubKey("SOFTWARE");